The top Zen’s will work only on top 370-470 boards, the middle will go to the middle boards because of vrm power. Nobody exactly promised that the new Zen’s with 12-16 cores will work on 320 chipset mobo.
How is that any different than now? If you buy a mid-end X370/X470 they're generally already struggling with sustained AVX loads with R7 2700X when overclocked.
What I think will happen is that if there's a 12 core the PPT (package power tracking) will be constrained under all core load (something akin to 130-150W) and when you have a good board the power limit will be lifted higher via BIOS to 200W or more. That's not going to change light load such as 4 core loads provided the power gating built into the Zen 2 chips is decent.